U.S. Pat. No. 4,299,865 discloses a two layered ceramic thermal barrier coating with an outer layer which has a very high porosity between 20 vol. % and 33 vol. % and a dense inner ceramic thermal barrier layer.
Thermal barrier coatings are substantially function layers and are attended to protect the substrate against excessive heat. The substrate has a sufficiently high mechanical strength and the thermal barrier coatings are likewise exposed to thermal strength and mechanical stresses and may fail as result of cracks.